Viruses, an important etiologic agent in infectious disease in humans and other mammals, are a diverse group of infectious agents that differ greatly in size, shape, chemical composition, host range, and effects on hosts. After several decades of study, only a limited number of antiviral agents are available for the treatment and/or prevention of diseases caused by viruses such as hepatitis B, influenza A and B and HIV. Because of their toxic effects on a host, many antiviral agents are limited to topical applications. Accordingly, there is a need for safe and effective antiviral agents with a wide-spectrum of anti-viral activity with reduced toxicity to the host.
Since the identification of the human immunodeficiency virus (HIV) as the causative agent of AIDS,.sup.36,46 the search for safe and effective treatments for HIV infection has become a major focus for drug discovery groups around the world. Investigations into the molecular processes of HIV have identified a number of macromolecular targets for drug design, such as HIV-1 reverse transcriptase (HIV-RT), protease and integrase enzymes, and regulatory proteins (e.g., TAT and REV). Other targets are enzymes which aid in virus attachment and fusion. HIV-RT is an essential enzyme in the life cycle of HIV, which catalyzes the transcription of HIV-encoded single-stranded RNA into double-stranded DNA. Furthermore, the RNA-dependent DNA polymerase function of HIV-RT does not have an analogous process in mammalian metabolism, and thus is a suitable target for a chemotherapeutic agent.
The hepatitis B virus (HBV) infects people of all ages. It is one of the fastest-spreading sexually transmitted diseases, and also can be transmitted by sharing needles or by behavior in which a person's mucus membranes are exposed to an infected person's blood, semen, vaginal secretions, or saliva. While the initial sickness is rarely fatal, ten percent of the people who contract hepatitis are infected for life and run a high risk of developing serious, long-term liver diseases, such as cirrhosis of the liver and liver cancer, which can cause serious complications or death..sup.1 The World Health Organization lists HBV as the ninth leading cause of death. It is estimated that about 300 million persons are chronically infected with HBV worldwide, with over 1 million of those in the United States. The Center for Disease Control estimates that over 300,000 new cases of acute HBV infection occurs in the United States each year, resulting in 4,000 deaths due to cirrhosis and 1,000 due to hepatocellular carcinoma..sup.2 The highest rates of HBV infections occur in Southeast Asia, South Pacific Islands, Sub-Saharan Africa, Alaska, Amazon, Bahai, Haiti, and the Dominican Republic, where approximately 20% of the population is chronically infected..sup.3
Hepatitis B virus (HBV) infection is currently the most important chronic virus infection, but no safe and effective therapy is available at present. The major therapeutic option for carriers of HBV is alpha interferon, which can control active virus replication. However, even in the most successful studies, the response rate in carefully selected patient groups has rarely exceeded 40%..sup.5,6 One of the reasons cited for interferon failure is the persistence of viral supercoiled DNA in the liver..sup.7 Clinical exploration of many promising antiviral agents such as nucleoside analogues is hampered because their aspecific body distribution leads to significant toxic side effects. Recently, however, a new nucleoside analogue, 2',3'-dideoxy-3'-thiacytidine (3TC), was discovered and found to be extremely potent against HBV replication with only minimal side effects..sup.8-10
Influenza is a viral infection marked by fever, chills, and a generalized feeling of weakness and pain in the muscle, together with varying signs of soreness in the respiratory tract, head, and abdomen. Influenza is caused by several types of myxoviruses, categorized as groups A, B, and C.sub.4. These influenza viruses generally lead to similar symptoms but are completely unrelated antigenically, so that infection with one type confers no immunity against the other. Influenza tends to occur in wavelike epidemics throughout the world; influenza A tends to appear in cycles of two to three years and influenza B in cycles of four to five years. Influenza is one of the few common infectious diseases that are poorly controlled by modern medicine. Its annual epidemics are occasionally punctuated by devastating pandemics. For example, the influenza pandemic of 1918, which killed over 20 million people and affected perhaps 100 times that number, was the most lethal plague ever recorded. Since that time, there have been two other pandemics of lesser severity, the so-called Asian flu of 1957 and the Hong Kong flu of 1968. All of these pandemics were characterized by the appearance of a new strain of influenza virus to which the human population had little resistance and against which previously existing influenza virus vaccines were ineffective. Moreover, between pandemics, influenza virus undergoes a gradual antigenic variation that degrades the level of immunological resistance against renewed infection..sup.4
Anti-influenza vaccines, containing killed strains of types A and B virus currently in circulation, are available, but have only a 60 to 70% success rate in preventing infection. The standard influenza vaccine has to be redesigned each year to counter new variants of the virus. In addition, any immunity provided is short-lived. The only drugs currently effective in the prevention and treatment of influenza are amantadine hydrochloride and rimantadine hydrochloride..sup.11-13 While the clinical use of amantadine has been limited by the excess rate of CNS side effects, rimantadine is more active against influenza A both in animals and human beings, with fewer side effects..sup.14,15 It is the drug of choice for the chemoprophylaxis of influenza A..sup.13,16,17 However, the clinical usefulness of both drugs is limited by their effectiveness against only influenza A viruses, by the uncertain therapeutic efficacy in severe influenza, and by the recent findings of recovery of drug-resistant strains in some treated patients..sup.18-22 Ribavirin has been reported to be therapeutically active, but it remains in the investigational stage of development..sup.23,24
While the search for viable therapeutics for treatment of both HBV and influenza infections has been moderately successful, therapeutic agents for HIV are severely limited. Furthermore, there are no known safe and therapeutic treatments for HBV, influenza and HIV. In HBV, with the possible exception of the drug 3TC, the use of nucleoside-based antiviral agents leads to toxicity, probably due to cross-inhibition of cellular mitchondrial DNA. Clearly, there is a need for a new class of antiviral agents which could minimize the toxicity associated with cross-inhibition. In influenza, amantadine and rimantadine have been shown to be moderately effective against only influenza A viruses; with amantadine having excessive side effects. Recently, strains of influenza A resistant to amantadine and rimantadine have been isolated. Accordingly, there is a need for new types of therapeutic antiviral agents against both influenza A and influenza B, as well as against HBV and HIV.